A broad issue for single-use filter users and suppliers across North America and Europe is the pre-use integrity testing of sterilizing filters in single-use systems. The subject has received considerable discussion lately, with differences in regulatory requirements and risk assessments being considered. The properties and suitability of gammaradiation sterilized single-use filtration systems, and the flexibility of these designs, were not considered when regulatory guidelines were written, but now provide additional options for manufacturers.

At the moment, there is no harmonized guidance on the pre-use integrity testing of sterilizing filters. FDA does not formally require it, stating in its aseptic processing guidance that "Integrity testing of the filter(s) can be performed prior to processing..." (1).

In contrast, the European Medicines Agency (EMA) states in EU GMP, Annex 1, "The integrity of the sterilized filter should be verified before use..." (2). So, while integrity testing after use is a common requirement, pre-use testing, whether before or after sterilization, is not a universal requirement.

This situation is further complicated in Europe where different national authorities apply EU GMP differently. Some European inspectors demand that pre-use, poststerilisation integrity testing be conducted because it is specifically required in EU GMP, while others are more flexible and do not require it.

The topic was discussed during a workshop at the May 2012 PDA–PIC/S conference in Geneva. Arguments were expressed both for and against testing, but insufficient time was allotted to fully explore the technical issues and no resolution could be drawn. This situation leaves manufacturers with inconsistent international regulatory expectations, and more work is needed to ensure that inspectors have a common understanding.

The issue is relevant to single-use disposable technology because EMA's published rationale for requiring pre-use, post-sterilization in integrity testing is based on the risks of filter damage or other loss of integrity during hightemperature sterilization processes, such as steam-in-place or autoclaving.

EMA states, "The filter sterilization process may be physically stressful for the filter. For example, high temperatures during the process may cause the filter to distort, potentially leading to fluid pathways that allow the passage of particles greater than 0.2 µm in size. The performance of a filter can improve with use, as particles begin to block individual pathways and remove larger pathways that smaller particles could successfully navigate" (3).

Although such filter distortions have been reported for some prototype membranes, commercialized sterilizinggrade filters are validated to maintain integrity and not undergo fluid path distortions (enlargement) during steam sterilization that might enable initial bacterial penetration. Furthermore, sterilization by gamma irradiation, which is typically applied to single-use filtration systems, can be considered a less stressful process (low heat, low pressure), and has not been implicated in the distortion of fluid pathways or loss of filter integrity so long as the membrane material is compatible with gamma radiation and that gamma dosage does not exceed specified maxima. These parameters are typically certified with every filter or single-use system lot. Many users question whether the possible risk of introducing contamination by performing a pre-use, poststerilization integrity test on a sterile filter in a gammasterilized single-use filtration system is higher than the risk of not performing the test.

The decision on whether or not to perform pre-use integrity testing should be made on a risk-assessment basis. Many users consider the filter manufacturer's integrity test prior to release to be sufficient confirmation that the filter is integral as made and is ready to use upon installation and sterilization. It is rare for users to experience filterintegrity failures post use, so any user pre-use testing, before or after sterilization, seems unnecessary. Experience with steam sterilized filters, however, suggests that most instances of post-use filterintegrity failures, when they do occur, are due to gross filter damage incurred during poorly controlled steam-in-place or occasionally autoclave cycles. Where steaming processes are not well characterized and controlled to avoid filter damage, damage (if it occurs) can be recognized prior to product filtration if a pre-use/poststerilization integrity test is performed.